1. Field of the Invention
The present invention relates to draglines and electric shovels, such as used in open cast (or open cut) mining, and more particularly to a method and apparatus for monitoring their boom load conditions. In what follows, the teachings are given for a dragline, it being understood that they apply mutatis mutandis to an electric shovel. A dragline is a piece of machinery used for scooping ground material by means of a bucket suspended from a boom.
2. Description of Related Art
FIG. 1 is a simplified diagram of a classical dragline 1. It comprises a base unit 2, a boom 4 having a proximal end 4a depending from the base unit and a distal end 4b fitted with a pulley (also known as a sheave wheel) 6, from which a bucket 8 is suspended by a metal (steel) cable, referred to hereafter as a hoist rope 10. The base unit 2 comprises an elevated structure 12 for passing the hoist rope 10 to the pulley 6. In the example, this structure includes a mast 14 at the front portion (seen from the pulley) and stays 16, from which the hoist rope 10 connects to a drive point in the base unit 2. The hoist rope is thereby driven from a motor drive in the base unit to raise and lower the bucket 8 as required. The boom 4 can be driven to swing in an azimuthal (horizontal) plane by an electric swing motor, and thereafter blocked at a set azimuth. In the example, the swing axis SW passes through the base unit 2, the latter being mounted on rotary platform.
The bucket 8 is pulled towards base unit 2 substantially along the ground (horizontal) plane by another metal (steel) cable, referred to hereafter as a drag rope 18, to carry out the scooping action. The drag rope 18 is attached at one end 18a to anchoring points 8a, 8b of the bucket, so that bucket's opening 8c is kept horizontal and facing the base unit 2. The other end of the drag rope is connected to an electrically-driven winch (not shown) within the base unit 2.
In operation, the distal end 4b of boom 4 is initially positioned over the zone where material 20 is to be scooped, typically 70-100 m above the ground. The hoist rope 10 is initially adjusted to suspend the bucket 8 vertically (dotted lines) with its opening 8c confronting piled material 20 to be scooped. The drag rope 18 is then driven to exert a tractive force TF which drags the bucket along the ground plane, thereby picking up material 20 through the opening 8c. At the same time, the portion of the hoist rope 10 hanging from the pulley 6 is lengthened to maintain the bucket suspended following along the horizontal path of the ground. After the bucket has been dragged over a certain distance, filled, and lifted at some distance above the ground by hoist rope, the boom 4 is swung to place the bucket over a dumping zone.
The bucket is then arranged to drop the material, e.g. by tilting the bucket using an appropriate mechanism.
The dragline constitutes a large scale structure, with a boom length of 80 meters or more and a bucket capacity of up to 250 tonnes. The forces exerted on the boom 4 result from a combination of the tractive force TF exerted by the drag rope 18 and the suspending force SF exerted on the hoist rope 10. In particular, the hoist rope transfers a very high load to the boom, notably during the hoisting phases for lifting and during swinging of the boom.
Under ideal operating conditions, the bucket 8, hoist rope 10 and drag rope 18 are maintained in azimuthal alignment with the principal axis of the boom BA (boom axis), i.e. the boom, hoist rope and drag rope are kept substantially in the same general plane, in alignment with the horizontal projection of the boom, as shown in FIG. 2. These alignments should ideally be maintained as the bucket 8 is pulled and the hoist rope 10 thereby subtends an evolving angle α (FIG. 1) with the vertical in the vertical plane containing the boom 4. In this way, the forces TF and SF on the boom are coplanar with the boom and exert a compressive force on its structure. In particular, the lateral stress LS on the boom, which would exert a lateral bending moment, is zero under those ideal conditions.
To meet the load demands, the boom 4 constitutes a complex mechanical structure made of steel, typically as a trellis box frame. The boom is a major limiting factor in the production rate of the dragline.
If the boom is overloaded, it will crack and cause downtime on the machine. If it is badly overloaded, it will cause complete failure of the structure. This is a major safety issue within a mine and can result in a fatal accident.
The boom 4 is usually specified for operation under these idealized working conditions, notably as regards its safe working load limits. With a proper control of the stresses within the boom structure, it would be possible to allow for a controlled overload of the dragline. This would give an improvement in output for a very low extra cost. Savings in terms of work efficiency under these circumstances can be typically on the order of hundreds of thousands of dollars per year per dragline.
It is known in the art to equip the boom with strain gauges at critical points to provide the dragline operator with a computer display showing stress-related parameters. This method, however, has the disadvantage of requiring rather complex calculations based on the boom structure characteristics, which may vary from one dragline to another.